Microstructure for vibratory gyroscope

Abstract: A microstructure for a vibratory gyroscope has a ring portion (14) supported so as to allow substantially undamped, high-Q radial vibration. The ring portion (14) is electrically conductive and comprises a charge plate (14) for a plurality of radially disposed charge conductive sites (13) around its perimeter for sensing radial displacements thereof. The ring (14), its support (15) and charge conductive sites (13) are formed within sacrificial moulds on one surface of a conventional silicon substrate which may comprise a monolithic integrated circuit.

Description: The present invention relates to a microstructure for a vibratory gyroscope. The field of rotation sensors is typified by devices applying vibrating mechanical elements (vibratory elements) as the rotation indication means. Numerous base forms of vibratory elements used in such vibratory rotation sensors are known including discs, tuning forks, cylinders and hemispherical shells. All such vibratory rotation sensors exploit the effects of the Coriolis force produced by the rotation of its vibratory element(s). The Coriolis force is known to influence detectably the orientation of a resonant vibratory pattern (standing wave) in a vibratory element in proportion with the rotation of the element but independent of the rate of rotation. Perhaps the most widely known form of vibratory rotation sensors employ three hemispherical shells as vibratory elements to detect rotation about three mutually orthogonal axes. Known in commercial avionics as Hemispherical Resonator Gyros (HRG), these devices provide a very high degree of accuracy and sensitivity at low rotation rates as required by inertial grade navigation systems. Other features of HRG include longer useful operating life, higher reliability and a more cost effective system than many alternative rotation sensing technologies for commercial and military aviation. Other vibratory rotation sensors are intended mainly for weapons guidance and control. Performance of this grade of sensor must withstand large linear and rotational accelerations characteristic of munitions. These types of sensors are more compact and less costly than HRG but are less accurate and sensitive at low rotation rates. Rotation sensing packages which are financially and functionally attractive in navigational avionics or weapons guidance are not so in other areas, for example commercial automotive electronics. Cost alone, aside from other factors such as size, mass and functional excesses or deficiencies would prohibit incorporation of such a device in an automobile for rotation sensing. EP-A-0461761 discloses a microstructure in accordance with the preamble of claim 1. The present invention seeks to provide an improved microstructure for a vibratory gyroscope. A microstructure in accordance with the present invention is characterised over EP-A-0461761 by the features specified in the characterising portion of claim 1. It is possible with the invention to provide a gyroscope having a moderate degree of accuracy, rotation detection about a single axis, minimal size and mass, efficient design for mass manufacturing and cost commensurate with an automobile's overall price structure, for use, for example, in vehicle navigation systems and chassis control systems such as active suspensions and active rear steering. The invention can provide a microstructure for a vibratory gyroscope of the type which senses rotation about an axis. The invention is defined in the claims. In a preferred method of forming a microstructure for a vibratory gyroscope, adhesion sites are established on a base for ohmic and structural interconnection to the microstructures. Consistent with the preferred embodiment, these adhesion sites would be established at the base of the central support member and charge conductive sites around the perimeter of the ring-like vibratory member. An adhesion site plating base layer may be established at the adhesion sites where metal microstructures will be formed and coupled. A sacrificial layer may be established upon the base to provide separation of all free standing portions of the microstructures from the base. In the preferred embodiment, this includes areas beneath the ring-like vibratory member and also beneath the plurality of spokes supporting the ring-like vibratory member from the central support member. A sacrificial plating layer is preferably deposited for the free standing portions of the microstructure. The microstructure is then defined out of the plane of the base by a mould having substantially vertical walls with respect to the plane of the base. The vertical walls of the mould expose therethrough the adhesion sites and the sacrificial plating base layer. The microstructure is preferably then electroformed within the confines of the mould to provide a homogenous metalised microstructure. The mould and any sacrificial layers are then removed to leave only the metalised microstructure intact. Preferably, the base to which the microstructure adheres comprises a prefabricated monolithic integrated circuit having at least a portion of control and readout circuitry contained therein. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figures 1a and 1b show plan and side elevational views of an embodiment of gyroscope; and Figures 2a to 2f show various steps of an embodiment of method for manufacturing the gyroscope of Figures 1a and 1b. Referring to Figures 1a and 1b, the embodiment of micromachined vibratory gyroscope 10 shown includes a base 12 which provides the foundation for the microstructure comprising the gyroscope. Conventional silicon substrates can provide adequate foundation for the gyroscope. Ring 14 of the gyroscope 10 is shown concentric with support hub 15. Eight equally distributed semi-circular spokes 16 couple ring 14 to hub 15. Hub 15 is coupled to base 12, whereas all lower surfaces of spokes 16 are spaced from base 12, as illustrated by gap 17 in Figure 1b. An alternate embodiment (not illustrated) takes the form of a cylinder attached at one end thereof to the base with the opposite end thereof free to support radial vibrations. However, as between these two exemplary embodiments, the former is preferred since all motion is in the plane of the base whereas the latter has some vibration perpendicular thereto and coupled to the base through the attached end of the cylinder, thus damping a portion of the energy not otherwise damped in the former embodiment. Angular gain (the amount of precession for a given rotation) is greater in the former embodiment where all of the energy is retained in one plane and is therefore preferred. An alternative spoke geometry has a meandering or 'S' shape in the plane of the base. A meandering shaped spoke may comprise radially aligned arc portions of a plurality of discontinuous concentric rings coupled radially at alternating pairs of arc ends by portions of a plurality of discontinuous radial members extending from a central hub to a vibratory ring. In the preferred exemplary embodiment having a hub and semicircular spokes as illustrated, eight spokes 16 are shown. A greater number of spokes may be used with greatly reduced sensitivity and accuracy of the device. However, it is generally true that any octonary multiple of spokes will provide the greatest sensitivity and accuracy possible for a hub and spoke support structure. Where an octonary multiple of spokes is used, damping effects upon a standing wave pattern introduced thereby are symmetrical about the ring. The frequency of the standing wave pattern at any location or alignment is therefore the same. Where a number of spokes not an octonary multiple is used, damping effects are asymmetrical and support standing wave patterns of varying frequencies depending on alignment. A standing wave pattern would have the tendency to drift to an alignment which offers the least resistance to its motion, that is the least damped alignment. Therefore, any non-octonary spoke structure introduces undesirable drift in the device. An arrangement of eight spokes is most desirable for reasons of overall minimal radial stiffness, minimal material usage, and simplification of manufacture for any given octonary multiple. Shown also in Figures 1a and 1b are a multiplicity of charge conductive sites 13, generally known as pick-offs or drive electrodes, disposed symmetrically around the outer perimeter of ring 14 and spaced therefrom by a gap 18. The arrangement of charge conductive sites 13 shown in Figure 1a is illustrative of at least one configuration adequate for detection and drive of a resonant standing wave pattern in ring 14. Reference is made to a specific control and readout circuit is disclosed in our co-pending European patent application no. 94200104.1. Additional charge conductive sites (not shown) may be added depending on the particular device application, precision requirements and associated circuitry. Base 12, while shown in the exemplary embodiment of Figures 1a and 1b as a conventional silicon substrate, may also comprise prefabricated circuitry as a monolithic integrated circuit. In fact, a preferred embodiment encompasses at least a portion of device circuitry being incorporated within base 12 and resistively coupled to the microstructures via exposed metalisation sites. It is anticipated that by providing at least a buffer circuit in proximity to each capacitive site 13 that signal integrity will be greatly improved over that obtainable using more remote circuitry, since the charge conductive sites register relatively small capacitive changes which are relatively easily attenuated. It is desirable in fabrication of the ring 14 to maximise its height perpendicular to the plane of base 12. This would increase surface area around the perimeter which provides larger capacitive charges at correspondingly larger charge conductive sites 13, thereby making the device less sensitive to attenuation of the signals at the charge conductive sites 13. To the extent that the height of the microstructures permits, it is desirable to increase the diameter of ring 14 since a larger diameter will decrease the in-plane natural frequency of the ring 14. Decreasing the natural frequency of ring 14 yields increased sensitivity and decreased drift since these two parameters are frequency sensitive. As ring diameter is increased, out of plane natural frequencies of the structure are decreased, thus narrowing the gap between the in-plane natural frequency and the out of plane natural frequency. It is desirable that any out of plane natural frequencies remain greater than the natural frequencies of the in-plane natural frequency on ring 14 at least to the extent that the out of plane natural frequency does not interfere with detection of the in-plane natural frequency via increased signal (in-plane) to noise (out of plane) ratio. For example, a frequency equal to or greater than the three decibel point on the in-plane resonant frequency response curve is desirable for any out of plane natural frequency of the structure. Relative to thickness of the ring walls, it is most desirable to fabricate a ring 14 having minimum width since this too lowers the in-plane natural frequency. Additionally, a less massive ring 14 requires less energy input to maintain resonance. However, the particular application for the device will limit the lowering of the in-plane resonant frequency. For example, when sensing rotation in a motor vehicle, it is not desirable to lower the natural resonant frequency of the ring 14 to the extent that road noise or other vibrations characteristic of automobiles would be introduced into the sensed signals. Referring now to Figures 2a-2f, a fabrication sequence for production of the free standing vibratory ring portion 14 of the rotation sensing device is described. Substrate 30 comprises a silicon substrate material which may comprise a prefabricated monolithic integrated circuit. Aluminium interconnection 32 is provided primarily for resistive contact to integrated circuitry contained within substrate 30 or comprises a portion of electrical traces for coupling to circuitry external to substrate 30. A passivation layer 31, such as conventional plasma deposited silicon nitride or a low temperature oxide, is provided on substrate 30 and contact holes are opened above all aluminium interconnection areas. Figure 2a illustrates substrate 30 having an aluminium interconnection 32 exposed through passivation layer 31 to form base 12'. Resistive and structural portions of adhesion sites are next established respectively above and adjacent the aluminium interconnections 32. The resistive portions function primarily to couple electrically the microstructure to the aluminium interconnections while the structural portions adjacent thereto are established for mechanical coupling. Preferably, a 100 nanometre barrier layer 33 of a titanium/tungsten alloy followed by a 200 nanometre plating layer 34 of gold are deposited by sputtering. A protective layer 45 of tungsten approximately 100 nanometres is then similarly deposited. Protective layer 45 prevents gold plating layer 34 from reacting with an aluminium sacrificial layer deposited after adhesion site definition processing and described below. Patterning is next performed to form the adhesion sites onto which the microstructures will be deposited. The tungsten protective layer 45 is etched away in a carbon tetrafluoride (CF 4 ) over all areas not defining an adhesion site. The gold layer is next patterned in geometrically similar manner using a wet chemical etch solution of potassium iodide (KI/I 2 ) after which the titanium/tungsten alloy is similarly patterned in a carbon tetrafluoride (CF 4 ) plasma. Gold is the preferred plating base layer since most electro deposits have excellent adhesion thereto, although other materials such as copper or nickel could be used. Titanium/tungsten alloy barrier layer 33 serves the dual purpose of being an adhesion layer for gold plating layer 34 and a diffusion barrier to keep the gold from reacting with aluminium interconnection 32. With the resistive and structural portions of adhesion sites defined, the next step is the formation of a sacrificial spacer layer 35 (Figure 2b). This layer substantially defines space 17 shown in Figure 1b between base 12 and bottom edges of ring 14 and spokes 16. An opening 36 is provided above adhesions sites through sacrificial spacer layer 35. Preferably, aluminium is deposited by electron beam evaporation as sacrificial spacer layer 35 to a thickness of approximately 3 micrometres. Standard wet chemical etching is then used to pattern the aluminium sacrificial spacer layer 35 from over the adhesion sites to establish opening 36 therethrough. Figure 2b illustrates the structure of a single adhesion site up to this point in the process. The next step in the process is to establish sacrificial plating base layer 37 for the free standing portions of the microstructures. Preferably, a 75 nanometre layer of tungsten is deposited as sacrificial plating base layer 37 to cover sacrificial spacer layer 35 and the adhesion sites. Sacrificial protective layer 38 of approximate thickness of 75 nanometres is next deposited over the same areas. This layer 38 preferably comprises aluminium. Sacrificial protective layer 38 is next removed from over the adhesion sites by wet chemical etching in standard aluminium etch to leave the adhesion sites covered with tungsten for the purpose of protecting the gold thereunder from resputtering during further processing. Sacrificial protective layer 38 is provided to protect sacrificial plating base layer 37 during further processing and as an etch mask in the removal of tungsten left over the gold adhesion site prior to an electroforming operation, described below. Figure 2c shows the structure up to this point of the process. Mould formation is next performed. A preferred material for mould layer 39 is polyimide which is first spun on to the existing materials and cured to a thickness of approximately 25 micrometres. Mould layer 39 is then patterned to establish the external shapes of the microstructures. Preferably, mask layer 40 comprises a 100 nanometre layer of SOG available from Futurex Corporation which is spun on to the top of existing mould layer 39 and is itself patterned to establish the desired external shapes of the microstructures. A conventional positive photo resist mask is used to pattern layer 40 using carbon tetrafluoride (CF 4 ) as reactive gas in a reactive ion etching process. Pure oxygen is then used as the reactive gas in a reactive ion etching process to etch mould layer 39 down to plating base protective layer 38, thus resulting in near vertical mould walls with minimal undercut. Alternatively, it is possible to use photo sensitive polyimide for layer 39 and ultraviolet exposure to form the mould. After mould definition, tungsten layers 37 and 45 over the gold adhesion sites are removed by wet chemical etching utilising protective layer 38 as a mask, and plating base protective layer 38 is then itself removed by standard aluminium etch from over sacrificial plating base layer 37 between walls defined in mould layer 39. Figure 2d shows the structure up to this point in the process. The next step in the fabrication sequence is electroforming of the microstructures. Preferably, a barrett sulfamate nickel process is used to form nickel microstructures. This process is characterised by low stress deposits and high replicating ability. Microstructures are electroformed to near the top of the mould by plating at a predetermined current density for a fixed time. Plating time required is calculated from known plating rates of the barrett sulfamate nickel plating process. While nickel is used in this exemplary embodiment, other materials including nickel alloys can be used in electroforming microstructures. Nickel-iron alloys, for example, exhibit higher Q than pure nickel and may be a desirable alternative material. Figure 2e illustrates a completed electroformed microstructure 50 with mould and sacrificial layers still in place. A final major step in the fabrication sequence is removal of the mould and sacrificial layers to release the microstructures. In this exemplary embodiment with the materials heretofore utilised, the mould is removed by standard O 2 plasma ashing and the sacrificial spacer layer is removed by wet chemical processing with a solution of potassium hydroxide and potassium ferrocyanate. Figure 2f illustrates the free standing microstructure 50 with mould and sacrificial layers removed. Figure 2f is further more specifically illustrative of ring 14', support 15' and portion of spoke 16' analogous to similar components in Figure 1a and 1b. For simplicity of description and illustration, none of the charge conductive sites 13 are shown in Figures 2a-2f. Charge conductive sites are electroformed in the same processing as the free standing ring 14' and support 15'. Location of charge conductive sites 13 would be as mentioned earlier adjacent to ring 14' separated by a gap 18, as illustrated in Figure 1b. As is common practice with sensors utilising resonant structures, vacuum packaging is employed to obtain high quality factor Q vibration. It is therefore desirable to vacuum package the structure in order to maximise quality factor Q of the structure. Exemplary vacuum packaging techniques contemplated include enclosing the entire chip in a vacuum package using well known hermetic sealing techniques for semiconductors or using a capping wafer and modified flip-chip technology to vacuum package only the free standing portions of the microstructures on the chip level. The reader is referred to the disclosures in United States patent application no. 055,880, from which this application claims priority.

Claims: A microstructure for a vibratory gyroscope comprising a silicon substrate base (12); a ring (14); and support means (15,16) for supporting the ring over the base in a substantially radially undamped manner; wherein the support means comprises a support member (15) substantially perpendicular to the base; and spoke means (16) coupling the support member and the ring so as to support the ring substantially coaxially with the support member and such that the ring is spaced from the base, characterised in that the spoke means comprises an octonary multiple of substantially equally spaced, substantially semi-circular and radially compliant spokes (16); in that the perimeter of the ring is substantially entirely charge conductive; and by a plurality of charge conductive sites (13) disposed radially adjacent the perimeter of the ring to form a plurality of capacitive sites therewith. A microstructure according to claim 1, wherein the spoke means (16) comprises a plurality of radially compliant spokes being substantially S-shaped from the support member (15) to the ring (14). A microstructure according to claim 1 or claim 2, wherein the spoke means (16) is of a dimension to provide high-Q radial vibration of the ring. A microstructure according to any preceding claim, wherein the ring (14), the support means (15,16) and the plurality of charge conductive sites (13) comprise nickel. A microstructure according to any preceding claim, wherein the base (12) is a monolithic integrated circuit comprising signal conditioning circuitry. A microstructure according to any one of claims 1 to 4, wherein the base (12) comprises signal conditioning and control means for establishing, maintaining and detecting the position of a standing wave pattern in the ring. A microstructure according to any preceding claim, comprising a vacuum packaging capping wafer. A microstructure according to any preceding claim, wherein the ring (14) and the support means (15,16) form an integral and substantially cylindrical structure.

Inventors: EDDY DAVID STURGE, PUTTY MICHAEL WILLIAM, EDDY, DAVID STURGE, PUTTY, MICHAEL WILLIAM
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1994
Main classification: G01C19
IPC codes: B81B3, B81C1, G01C19, C23C14, G01P9